Recent elucidation of opioid peptides in invertebrate tissues has necessitated studies in their binding properties. Currently, it is believed that there are five distinct opiate receptor subtypes (mu, delta, kappa, epsilon and sigma) based on pharmacological studies and four specific binding sites on the basis of receptor binding in vertebrate species. Limited studies performed in invertebrates indicate receptor types analogous to that of the vertebrate mu and delta. Tissues of the earthworm lumbricus terrestris contain both leu-and met-enkephelin as well as B-endorphin. The distribution of these peptides is not only in neuronal tissues (supra- and subpharyngeal ganglia and ventral nerve fiber) but also in several peripheral sites (intestinal and seminal vesicle tissues). As a result, peripheral sites (intestinal and seminal vesicle tissues). As a result, earthworm tissues serve as an excellent model for the study of opiate receptors in invertebrates and may provide valuable information to our understanding of the heterogeneous nature of these receptors in higher vertebrates and the molecular basis for the difference in receptor subtypes. the aim of this study is to initiate binding studies for the opiate receptors in earthworm tissues. The long-term objective is to describe the opiate receptor subtypes and the relationship of these subtypes found in neuronal and peripheral tissues. It is anticipated that through this study possible interactions of regulatory activities of ions and guanylnucleotide will be established, since guanosine triphosphate (GTP), a known obligatory cofactor for the hormonal activation of adenylate cyclase activity, selectively effects dopaminergic and opiate receptors. In invertebrates, including the earthworm, dopaminergic neurons are closely associated with opioid peptidal activities.